The invention relates to a closed press section in a paper machine, comprising a compact combination of press rolls in which some of the rolls form press nips with each other. The web has a closed draw between the press nips and is supported on a face of at least one fabric. The press section further comprises a center roll, in connection with which at least one press nip is formed. A closed loop of a transfer band is passed around the center roll. The web is transferred on an outer face of the transfer band after the last press nip in the compact combination of rolls, as a closed and constantly supported draw onto a drying wire or an equivalent fabric in the drying section following the press section.
The invention also relates to a method for dewatering a web in a press section in a paper machine including a compact combination of press rolls having at least one press nip defined between a center roll and a first press roll and a plurality of fabrics for supporting a web in its run through the press section.
A particular problem in prior art press sections in paper machines is caused in the area in which a web that has passed through the nips in the press section is detached from a smooth-faced roll in the press, in particular from the center roll, and transferred to the drying section of the paper machine. This problem is accentuated with increasing running speeds of paper machines and/or when the paper web is very weak, e.g. when a maximal bulk is desired. In such a case, the pressing is carried out with very low nip loads, and the wet strength of the web remains low. Problems arise when the web is separated from the smooth-faced roll because a high tensile strain is applied to the web. Other problems arise in the area where the web is transferred from the press section to the drying section and it must run a short distance as a free and unsupported draw unsupported by a roll face or by a fabric. When the running speed of the paper machine is increased, there is a high risk that the paper web will be torn in these problem areas.
In conventional press sections, the web should preferably be detached from the center roll so that it runs to the drying section in a run which is as straight as possible. In this preferred situation, the area of transfer of the web from the press section to the drying section has constituted a significant bottleneck in attempts to increase the running speed of the paper machine.
In prior art press sections, attempts have been made to solve the problems discussed above, e.g., by means of the press sections described in U.S. Pat. Nos. 4,359,827 and 4,359,828. In the press sections described in these patents, the web is not placed in direct contact with a face of a center roll in the press section, but a porous belt carries the web around the center roll. The belt corresponds to the length of the roll, i.e., to the working width of the machine, and while guided by guide rolls, supports the web thereon during the transfer run from the press section to the drying section. However, it is a substantial drawback of the press sections in these U.S. patents that the porous band utilized therein loses some of its porosity in the nips in the press section, in which it is compressed to some extent. It is a second substantial drawback in these press sections that, at high temperatures, the band may lose most of its porosity, and in some extreme cases it may even melt.
Attempts are also made in the prior art to employ high temperatures in the press section in order to intensify the dewatering. It is a further drawback in the press sections of the above-referenced U.S. patents that the possibility of cleaning the porous belt is very poor. In the nips in the press section, the pores in the belt tend to be blocked, and the descriptions in these U.S. patents do not teach or suggest any means for conditioning and cleaning the band.
In Finnish Patent Application No. 885737 (filed Dec. 9, 1988), attempts have been made to provide a solution by whose means the drawbacks described above are avoided. The embodiments described in that patent application permit an increased running speed of a paper machine and ensures a problem-free transfer of the web from the press section to the drying section. In view of achieving the above, in these patent application, an endless metal band is passed over the center roll. The metal band is formed as a closed loop by means of guide and tensioning rolls and the closed loop is passed from the center roll to the beginning of the drying section. Thus, the web is transferred from the press section to the drying section while supported by this closed loop.
It is general aim among those skilled in the art to improve the dewatering capacity of presses in the press section of a paper machine. If the moisture content of a paper web can be minimized in the press section, this results in considerable economies in the costs of paper manufacture, because the less wet the paper web is when it arrives in the dryer section from the press section, the lower the amount of energy consumed in the drying section. It can be considered a rule of thumb that, if the moisture content of the web in the press section can be lowered by one percentage unit, the consumption of energy in the drying section is about four percent lower, which translates to considerable economies in cost. The dewatering capacity is generally improved by raising the pressing temperature of the paper web.
In the constructions commonly employed in prior art press sections, the center roll in the press constitutes an object of development. This is because of the material of the center roll which is commonly some suitable rock, for example granite. It is well known that rock rolls are quite sensitive to extensive and sudden changes in temperature, and the effects of such changes may be quite detrimental to the smooth operation of the paper machine. Attempts have been made to develop suitable substitutes for granite rolls. However, it is difficult to make a suitable face for a center roll, and, moreover, the making of the face restricts the choice of the material for the rest of the roll.
In addition, different paper qualities require a different coating and frequently also a different process for the manufacture of the roll coating. Often, it is necessary to manufacture different paper qualities out of different raw materials by means of the same paper machine. A change in quality would also require change of center roll or at least of its coating. A center roll is, however, an expensive and heavy component, and its replacement requires a long and costly standstill of the paper machine.
If a center roll is provided, e.g., with a welded coating or if the coating is elastic, such as rubber-like, the device intended for heating of the paper web must necessarily be placed above the web if it is desirable to provide such a heating device in the construction before an additional press nip in the press not formed against the center roll. However, before the additional press nip in the press, there is hardly any space available for an efficient device that raises the temperature of the web and regulates the temperature profile. It is a drawback of a heater placed in this location that it causes contaminants to gather and fall down onto the paper web.
An attempt has been made partly to solve the problems discussed above by means of the method and the device described in Finnish Patent Application No. 891343 (filed Mar. 21, 1989). In the method described in FI 891343, a transfer band formed as an endless loop by means of tension and guide rolls, is passed over the center roll in the press section. The transfer band extends to outside the area of the press rolls and the paper web is transferred to run onto the transfer band. The paper web is heated in the area of the loop of the transfer band outside the press rolls.
The device in accordance with FI 891343 does not, however, solve the problems discussed above related to the detaching of the paper web from the face of the center roll and to its further transfer.
One particularly difficult problem, which was mentioned above, arises from the fact that different paper qualities are often manufactured by means of the same paper machine, e.g., depending on the market situation, the orders, or on the type of raw material that is available. The center rolls currently in operation and the belt solutions related to them do not permit a rapid and smooth change of paper quality produced by the press section. The change of quality should ideally take place quite rapidly, because any standstill time causes considerable economic losses. In the press sections presented in the prior art, adequate consideration has not been given to quick replacement of the belt running around the center roll or of the other press fabrics and press rolls in the prior art solutions.
Moreover, in prior art center-roll/transfer-belt arrangements, particular attention has not been paid to proper conditioning of the transfer belt running around the center roll or in other respects to safety and optimization of the transfer of the web from the transfer belt.